diff --git a/.changeset/fix-worker-count-env-validation.md b/.changeset/fix-worker-count-env-validation.md
new file mode 100644
index 0000000..cea33fb
--- /dev/null
+++ b/.changeset/fix-worker-count-env-validation.md
@@ -0,0 +1,5 @@
+---
+"codemap": patch
+---
+
+Reject malformed `CODEMAP_PARSE_WORKERS` values (e.g. `2abc`, `1.5`) instead of silently truncating with `parseInt`.
diff --git a/src/worker-pool.test.ts b/src/worker-pool.test.ts
new file mode 100644
index 0000000..88b8a2f
--- /dev/null
+++ b/src/worker-pool.test.ts
@@ -0,0 +1,24 @@
+import { describe, expect, test } from "bun:test";
+
+import { parseParseWorkerCountOverride } from "./worker-pool";
+
+describe("parseParseWorkerCountOverride", () => {
+  test("accepts valid decimal integers", () => {
+    expect(parseParseWorkerCountOverride("2")).toBe(2);
+    expect(parseParseWorkerCountOverride("32")).toBe(32);
+    expect(parseParseWorkerCountOverride("999")).toBe(32);
+  });
+
+  test("rejects malformed or non-positive values", () => {
+    expect(parseParseWorkerCountOverride("2abc")).toBeNull();
+    expect(parseParseWorkerCountOverride("1.5")).toBeNull();
+    expect(parseParseWorkerCountOverride("0")).toBeNull();
+    expect(parseParseWorkerCountOverride("-1")).toBeNull();
+    expect(parseParseWorkerCountOverride(" 2")).toBeNull();
+  });
+
+  test("treats unset or empty as no override", () => {
+    expect(parseParseWorkerCountOverride(undefined)).toBeNull();
+    expect(parseParseWorkerCountOverride("")).toBeNull();
+  });
+});
diff --git a/src/worker-pool.ts b/src/worker-pool.ts
index 140b9e3..3f3e289 100644
--- a/src/worker-pool.ts
+++ b/src/worker-pool.ts
@@ -20,14 +20,25 @@ const WORKER_URL_NODE = new URL(
   import.meta.url,
 );
 
+const PARSE_WORKER_COUNT_RE = /^\d+$/;
+
+/** Returns clamped override [1, 32], or `null` when unset/empty/invalid. */
+export function parseParseWorkerCountOverride(
+  env: string | undefined,
+): number | null {
+  if (env === undefined || env === "") return null;
+  if (!PARSE_WORKER_COUNT_RE.test(env)) return null;
+  const parsed = Number(env);
+  if (!Number.isSafeInteger(parsed) || parsed < 1) return null;
+  return Math.min(parsed, 32);
+}
+
 // Override via `CODEMAP_PARSE_WORKERS` (clamped [1, 32]); default formula unchanged when unset.
 function resolveWorkerCount(): number {
   const env = process.env.CODEMAP_PARSE_WORKERS;
+  const override = parseParseWorkerCountOverride(env);
+  if (override !== null) return override;
   if (env !== undefined && env !== "") {
-    const parsed = Number.parseInt(env, 10);
-    if (Number.isFinite(parsed) && parsed >= 1) {
-      return Math.min(parsed, 32);
-    }
     console.error(
       `[worker-pool] ignoring invalid CODEMAP_PARSE_WORKERS=${JSON.stringify(env)} (expected positive integer ≤32)`,
     );